Small image sensors continue to improve in cost and performance and have become ubiquitous in smart phones, notebook computers, tablets and many other devices. At the same new device types such as headsets, glasses, dashboard cameras, and autonomous vehicles continue to emerge. The common CMOS (Complementary Metal Oxide Semiconductor) image sensor that is used in most digital cameras has an array of photodetectors. Usually there is one photodetector for each pixel. The sensor is well suited to capture and measure visible light.
The same sensor is also able to capture and measure NIR (Near Infrared) light. As a result, new applications are being developed to exploit this property. Biometric authentication and depth cameras, for example have been developed to use NIR. NIR has a benefit of revealing features that are not visible in visible light. Such features may reflect NIR but not visible light or the system may incorporate invisible, NIR illumination that does not distract or otherwise influence the user.
Every image sensor technology that relies on incident photons must be of a certain size in order to capture enough photons to make an accurate measurement. A smaller sensor captures fewer photons because fewer photons strike the smaller area. A larger sensor is more accurate but it requires more volume for housing and is more expensive to make. A further limitation of sensors comes in quantum efficiency. As the size of the pixel approaches the wavelength of the light being detected, the response of a CMOS pixel diminishes.
The loss of quantum efficiency may be compensated for by increasing illumination of the scene with a lamp or flash, increasing the collection or exposure time, or using faster or brighter optics to focus the light.
The light collection efficiency of a CMOS silicon photodetector also suffers from a relatively high reflection of photons at the air-silicon interface from the ambient into the sensor. This is because the refractive index of air is very different from that of silicon. Flat silicon surfaces have a light reflectivity in air of 35-40%. This may be compensated for by using antireflection coatings on the sensor.
Transparent quarter wavelength layers of SiOx, TiOx, or SixNy with intermediate or gradient refractive indices work well as antireflection (AR) coatings. AR coatings may be formed by single or multiple layer film deposition through various processes such as plasma enhanced chemical vapor deposition and sputtering. These coatings have resonant structures and work very well within the designed spectral range and angles of incidence. Some also provide hardness to protect the underlying sensor and electronics.